The present invention relates generally to the problem of robotic and/or remotely-controlled manipulators and, more particularly, to a simple, compact device which provides general wrist-like motions useful for a wide range of torque levels.
The development of robotic manipulators has undergone rapid development in the past twenty years. However, most robots comprise an arm made of a series of one-axis "hinge" joints (similar in principle to the human elbow joint) and rotational joints having a single axis of rotation. At the end of this arm is mounted the tool or manipulating device. Such arrangements are useful for a wide variety of applications. Indeed, there exist automobile assembly plants which are almost entirely dependent on such manipulators.
Despite a considerable degree of success, however, there are definite limitations to the capabilities and efficiency of such robotic manipulators. A major problem for general application is the need to use a rotational joint at the bottom of the arm to allow a large volume of space to be accessible using a minimum of joints and links (a link is the member between joints). However, use of a rotational joint means that any control or power conduits (generally either electric or hydraulic lines) traversing the rotational joint to reach joints further along the arm will be twisted on rotational motion. Such twisting leads to complications such as conduit wear, rotation limits, slip rings, hydraulic swivel joints, and other complications. The result is that the capability and reliability of the robotic arm are compromised.
A further problem concerns the difficulty of obtaining fluid and efficient motion with an arm made up of one-axis hinge and rotational joints. It is true that any motion can dissected into simple bending and rotational movements. However, the practical issue is to produce the desired types of movement with a minimum of complexity (complexity being inversely proportional to reliability). A useful illustration to contemplate is the striking of a tennis ball with a racket. The human body has four main joints involved in producing the appropriate racket motion, the wrist, forearm, elbow, and shoulder. (The structure of the wrist-forearm-elbow structure allows perhaps 120.degree. rotation of the wrist relative to the elbow. For the present discussion, this is considered another joint.) In a tennis swing, the shoulder joint lifts the arm from the side of the body and swings the arm back and forth at an attitude giving maximum leverage and power--a two-axis motion which is provided by a structure similar to a ball and socket joint. The elbow joint provides one-axis rotational motion which mainly positions the tennis racket at a point where it can impact the ball. The wrist, however, controls the orientation and detailed motion of the racket (e.g., to put spin on the ball) by a combination of rotation along the forearm axis (made possible by the overall wrist-forearm-elbow structure as described above) and rotation about the other two rotational axes. The resulting structure is capable of near-miracles of ball control, making the game of tennis thrilling to watch.
Consider, however, a hypothetical situation where all arm joints are one-axis hinge joints, except for one rotational joint of restricted rotation between the elbow and wrist. The shoulder joint is again used to raise the arm from the side of the body. However, that is the only motion of which it is capable, so it can play no further role in the swing. In particular, it provides a weak anchor for the subsequent swing rather than a dynamic connection to the mass and power of the body provided by a real shoulder. The elbow joint is still a hinge joint, and the best opportunity we have to obtain leverage for the swing. We will therefore assume that the elbow is oriented just as it is when one lifts an arm to the side from the shoulder. The elbow is then capable of giving a back-and-forth motion in a plane which is, depending on the shoulder position, approximately parallel to the ground. The hand now grips the racket in the conventional fashion, connected to a wrist that allows pivoting around only one axis (again assume the major axis of a real wrist, so that the axis of rotation of the wrist is in the plane of the palm and perpendicular to the forearm) and a forearm which allows about 120.degree. of rotation about its own axis. A few moments of contortions with these limitations in mind will quickly convince the reader that the structure described above is nearly useless when required to produce the complex dynamic motions required to play tennis properly, even though it is perfectly suitable to a small set of well-defined manipulations, such as performing simple assembly steps in a manufacturing environment under close human observation and control.
It is possible to design an arm out of one-axis hinge joints and restricted in-line rotational joints that allows motions suitable for playing tennis, but such an arm is extremely complex, presenting many difficulties in developing a suitable control system. The combination of a desired motion from one position may require use of a non-obvious path, resulting in inhibited and rather strange motions. (Mathematically this is similar to the property of frustration in spin glasses, wherein spins in a material interact in such a manner as to get `stuck` in an orientational pattern which is not the thermodynamic ground state of the system.)
An example of the difficulties which can arise in even a simple manipulator based on one-axis joints is provided by an altazimuth telescope mount. Such a mount has a first axis of rotation perpendicular to the ground on which the mount sits and a second axis of rotation perpendicular to the first which allows for rotation limited between 0.degree. (the horizon) and 90.degree. (straight overhead). (Such limitations are often required because of the size and design of the telescope itself.) It is clear that such a mount will allow a telescope mounted on a shaft turning with the second axis of rotation complete access to any point in the sky. It is also common to put motor drives on both axes to allow the telescope to follow the motion of the stars in the sky, to prolong the observation period. As neither axis of the altazimuth telescope mount is parallel to the Earth's axis of rotation, however, the rate at which the first and second axes are driven is a strong function of where the telescope is pointing. This rate is found using a coordinate transformation matrix, and the required control is usually provided by a computer system. The problem is that the transformation function has a singularity. Consider the case where the telescope is following a star which will pass directly overhead at midnight. At 11:59:59 PM the coordinates of the telescope mount are (36.degree., 89.99.degree.) (first axis, second axis positions). At 12:00:01 AM the coordinates required to follow the sidereal motion of the star are (324.degree., 89.99.degree.). In other words, even though the mount allows access to every part of the sky, extraordinary rapid motion is required to follow an object which passes nearly overhead. This is a very simple example of a problem which becomes an omnipresent concern when complex manipulators made of simple joints are considered.
The point of the above discussion is that the problem of design of robotic manipulators suitable for a wide range of functions, especially when the functions may involve uncontrolled classes of real-world environments and situations not thought of during design, requires a new view toward design of powered joints. The desire for reliable and smoothly functioning manipulators, combined with the limitations of present approaches show a clear need for simple multi-axis joints capable of being combined into manipulators suitable for a wide range of applications. A second need is for powered joints of minimal dimensions, so that the manipulator can be used in existing environments having a limited access, such as sampling and removing waste material from a radioactive waste storage tank. The picture of a computer-controlled snake comes to mind as the desired structure. That is, a long thin manipulator made of a nearly continuous backbone of two-axis angular joints and a `musculature` anchored on the backbone itself to provide the leverage required to exert the required forces for movement and operation.